At the present time raw coal contains three forms of sulfur, i.e., organic, sulfate and pyritic. A portion of this pyritic sulfur can be gravimetrically separated. With the EPA standard of 1.2 pounds of SO.sub.2 emission per million (MM) BTU there has been considerable difficulty in removing sulfur to meet this standard.
Coal burned by coal fired electric utility plants is the principal source of sulur oxide air pollution. Thus there is a need to remove as much sulfur as possible from the coal. In a typical year 1974, electric utilities burned 390 million tons of coal with an average sulfur content of 2.2%.
Suflur can be removed from coal by three methods - (1) Gravimetric separation (heavy organic liquids) of pyritic sulfur prior to combustion, (2) Flue gas scrubbing to remove SO.sub.2 such as in U.S. Pat. No. 3,752,877, and (3) Coal conversion to a clean fuel by gasification, liquefaction or chemical extraction.
Of all these methods pyritic sulfur removal is cheapest, but organic sulfur cannot be removed and the EPA limits are still difficult to meet.
Pyritic sulfur occurs naturally in coal as pyrite and as a discrete particle. It has a specific gravity of about 5.0 as compared to coal which is only 1.7 as a maximum. Gravimetric separation will reduce the sulfur significantly, but leaves behind organic sulfur and sulfate containing sulfur (usually less than 0.05%).
As a general rule, raw coal contains on the average 14.0% ash, 1.91% pyritic sulfur and 3.02% total sulfur (Bureau of Mines R.I. 8118/1976).
This invention is directed to a novel process of removing continuously as high a percentage of pyritic sulfur as possible at low cost. Even good gravimetric separation will only reduce pyritic sulfur from 2.01% to 0.66% or 65% reduction leaving 0.66% pyritic which when added to other sulfur will still be excessive by EPA standards. This invention is directed to removing the pyritic sulfur magnetically either as a separate step or in combination with other sulfur removal techniques depending on process considerations and cost.
One example of prior art technology as applied to Clay is U.S. Pat. No. 3,676,337, but this is not really a continuous process but calls for periodic flushing.
This application is believed to have application in the removal of any ferro magnetic or paramagnetic material from an ore or raw material. In this connection, the removal of iron from bauxite or shales from coal may be other applications.